Yes, very easy to germinate, but can be time consuming to extract from the fruit. You can just bury a whole fruit and you'll get a bunch of seedlings germinating which you can then divide and pot up separately.
Ok I just have to wait for the good moment to harvest the fruits. Do they stay green when mature or do they change color ?
They should turn a bit more bright lime green when ripe, although the fruit remains quite hard. Eventually, when the season changes they will all fall off the tree. I would suggest collecting the fruit for seed at that time. There is a lot of interesting historical/ethnobotanical information on this tree. Apparently the Osage Indians of N. America prized the wood of the Osage Orange tree for making bows due to its long, curved growth habit and tensile strength. The tree is related to fig, mulberry, jackfruit, and other members of the Moreaceae family.
Ok I think I'll wait the end of September to go back to this tree and check if the fruits fall from the tree